On Wed, 20 May 2020 13:26:57 +0200,
Rik van Riel wrote:
>
> After a few more weeks of digging, I have come to the tentative
> conclusion that either the XHCI driver, or the USB sound driver,
> or both, fail to handle USB errors correctly.
>
> I have some questions at the bottom, after a (brief-ish) explanation
> of exactly what seems to go wrong.
>
> TL;DR: arecord from a misbehaving device can hang forever
> after a USB error, due to poll on /dev/snd/timer never returning.
>
> The details: under some mysterious circumstances, the PCM290x
> family sound chips can send more data than expected during an
> isochronous transfer, leading to a babble error. Those
> circumstances seem to in part depend on the USB host controller
> and/or the electrical environment, since the chips work just
> fine for most people.
>
> Receiving data past the end of the isochronous transfer window
> scheduled for a device results in the XHCI controller throwing
> a babble error, which moves the endpoint into halted state.
>
> This is followed by the host controller software sending a
> reset endpoint command, and moving the endpoint into stopped
> state, as specified on pages 164-165 of the XHCI specification.
>
> However, the USB sound driver seems to have no idea that this
> error happened. The function retire_capture_urb looks at the
> status of each isochronous frame, but seems to be under the
> assumption that the sound device just keeps on running.
>
> The function snd_complete_urb seems to only detect that the
> device is not running if usb_submit_urb returns a failure.
>
> err = usb_submit_urb(urb, GFP_ATOMIC);
> if (err == 0)
> return;
>
> usb_audio_err(ep->chip, "cannot submit urb (err = %d)\n", err);
>
> if (ep->data_subs && ep->data_subs->pcm_substream) {
> substream = ep->data_subs->pcm_substream;
> snd_pcm_stop_xrun(substream);
> }
>
> However, the XHCI driver will happily submit an URB to a
> stopped device. Looking at the call trace usb_submit_urb ->
> xhci_urb_enqueue -> xhci_queue_isoc_tx_prepare -> prepare_ring,
> you can see this code:
>
> /* Make sure the endpoint has been added to xHC schedule */
> switch (ep_state) {
> ...
> case EP_STATE_HALTED:
> xhci_dbg(xhci, "WARN halted endpoint, queueing URB anyway.\n");
> case EP_STATE_STOPPED:
> case EP_STATE_RUNNING:
> break;
>
> This leads me to a few questions:
> - should retire_capture_urb call snd_pcm_stop_xrun,
> or another function like it, if it sees certain
> errors in the iso frame in the URB?
I guess it makes sense, yes.
> - should snd_complete_urb do something with these
> errors, too, in case they happen on the sync frames
> and not the data frames?
Ditto, the error can be handled similarly.
> - does the XHCI code need to ring the doorbell when
> submitting an URB to a stopped device, or is it
> always up to the higher-level driver to fully reset
> the device before it can do anything useful?
> - if a device in stopped state does not do anything
> useful, should usb_submit_urb return an error?
> - how should the USB sound driver recover from these
> occasional and/or one-off errors? stop the sound
> stream, or try to reinitialize the device and start
> recording again?
When snd_pcm_stop_xrun() is called, it stops the stream and sets in
XRUN state. Then the application receives -EPIPE error upon the next
access, and the application needs to re-setup the stream and restart.
Takashi
>
> I am willing to write patches and can test with my
> setup, but both the sound code and the USB code are
> new to me so I would like to know what direction I
> should go in :)
>
> --
> All Rights Reversed.
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>
